1. Field of the Invention
The invention pertains to sinter molding of plastic articles, more particularly a method for molding or forming without pressure, as in open molds. The manufacture of self-supporting plastic liners for fiber drums is representative of the uses to which the method of the invention is directed.
2. The Prior Art
It has been proposed to produce plastic articles by sinter molding, as by spraying or scattering material on a heated form or mold. However, the prior methods and apparatus do not achieve consistently uniform results, particularly in forming receptacle walls. Proposed prior methods and apparatus are principally deficient in that they do not provide regular, co-ordinated deposition and fusion for progressive assimilation along the form or mold surface, so that material is erratically distributed, the condition being aggravated by spalling and slumping. The resultant articles are generally characterized by non-homogeneity, voids, thin spots, porosity, poor physical properties, section and thickness irregularity, and like defects. Attempts to overcome these defects by excess charging, centrifuging and similar expedients, have proved ineffective, because they do not mitigate the effects or irregularity to incipient sintering, the material being only moderately fluent when sintered. Further, such expedients prove generally uneconomical, in that a substantial excess of the material must be deposited, involving bulky apparatus, protracted deposition time, and recirculation or reclamation of a large proportion of material which does not assimilate upon initial deposit.
Large receptacles, such as drum liners, have generally been produced by working latex on mandrels, or by vacuum-forming elastomer sheet stock, and then vulcanizing or curing. Such methods have proved costly and not entirely satisfactory. Limitations of material selection, high manufacturing cost and limited adaptability of the process often preclude obtaining the most suitable chemical and physical properties. Considerations of economy in manufacture and installation, and conservation of fill-space in the lined article generally dictate that the liners be only so thick as is necessary for desired protection, on the order of one thirty-second to one-sixteenth inch thick. With prior art materials and manufacturing techniques, large liners having a thickness of the order indicated are generally not self-supporting, are difficult to handle and ship, often do not fit the drum with sufficient accuracy, and are therefore excessively subject to damage in handling, shipment and in use. Losses, delays and high cost are frequently aggravated by the circumstance that specialized sources of liner supply may be remote from the site at which the drums or other finished articles are manufactured.